1. Field of the Invention
The present invention relates to an improvement of a speed control apparatus for an elevator, and more particularly to an improved elevator speed control apparatus having a speed detector enabling the accurate and reliable detection of a traveling speed of the elevator, whereby the smooth speed control of the elevator can be realized.
2. Description of the Related Art
In general, a speed control apparatus for an elevator is constructed by a feed-back control system, in which there are included a speed instruction unit for producing a traveling speed instruction and a speed detector for detecting an actual traveling speed thereof. A control unit obtains a difference between the speed instruction and the actual traveling speed detected and produces a torque instruction signal in accordance with the difference. A power convertor for supplying electric power to a driving motor of the elevator is operated on the basis of the aforesaid torque instruction signal, whereby the actual traveling speed of the elevator can be controlled in accordance with the speed instruction.
As a recent trend in this field, for the rotational speed control of a driving motor of an elevator, there is often adopted a so called vector control method, which makes it possible to control the rotational speed of the motor with the very high accuracy. Accordingly, a speed detector is required to have the high detection accuracy well-matched therewith. Usually, a speed detector using a rotary encoder is employed in order to meet the requirement of the highly accurate speed detection.
The rotary encoder generates a pulse train in response to the travel of an elevator, a pulse width of each pulse of which is measured by using a clock pulse signal with a sufficiently high frequency. A traveling speed of the elevator is obtained by dividing a traveling distance of the elevator per pulse from the rotary encoder by the pulse width measured as above.
In the speed detector of the rotary encoder type as mentioned above, the traveling speed can be detected with the very high accuracy so far as normal pulses continue to be produced. However, when a defective or abnormal pulse occurs in the rotary encoder, the detected value of the traveling speed is influenced thereby and widely fluctuates. Especially in an elevator control apparatus having a power convertor which repeats the conduction and interruption of a high voltage or a large current, defective pulses are apt to be produced, because large noise is induced in a great deal.
The fluctuation in the detected value of the traveling speed in turn causes the fluctuation in the aforesaid torque instruction signal, with the result that the smooth speed control of the elevator is damaged and resultantly the rough travel thereof makes passengers uncomfortable very much.